The objective of this proposal is to identify and characterize prostatic epithelial stem cells and to define the regulators of their growth and differentiation. The isolation of stem cells is of relevance to prostate carcinoma and benign prostatic hypertrophy, as both diseases are considered to arise from the aberrant proliferation of stem cells. We have identified a quiescent, slow-cycling population of cells with high proliferative potential in the proximal region of prostatic ducts. This region also contains cells with the greatest proliferative potential and the ability to form large, branched three dimensional glandular structures in collagen gels. In addition, we find that c-kit (receptor for stem cell factor, SCF), is expressed on 25% of basal cells while being completely absent from luminal cells. As many stem cell populations express c-kit, are quiescent and have innate high proliferative potential, we propose that a small number of prostatic stem cells are located in a 'niche' in the proximal region in the basal compartment and express c-kit on their surface. We further propose that the regulation of stem cell quiescence and proliferation is mediated by reciprocal interactions between transforming growth factor-B (TGF-B), which inhibits growth and SCF, which promotes growth. These hypotheses will be tested in three aims that exploit our ability to isolate subsets of viable basal cells expressing a6 integrin and c-kit and to test their engraftment in two in vivo models. Aim I will identify and separate subsets of basal cells most enriched for cells with stem cell properties. The proliferative potential of these subsets and their ability to generate glandular structures will be determined. In addition, culture conditions that allow their expansion while preventing their differentiation will be identified. New surface markers that will allow further selection and purification of stem cells will be sought. Aim 2 will quantitate the ability of the stem cell containing subsets to proliferate in vivo, to repopulate their organ of origin and to give rise to differentiated progeny. This will be done by implanting cells under the renal capsule and by intraprostatic inoculation in a novel transplantation model. Aim 3 will determine whether the balance between quiescence and proliferation is mediated by opposing effects of TGF-B and SCF. The sites of active TGF-J3 generation along the proximal-distal ductal axis will be identified under different hormonal conditions and the ability of TGF-13 to regulate c-kit and SCF expression studied. These experiments will define the subset of proximal cells in which prostatic stem cells reside and identify the manner in which their growth is regulated.